Picture transmitting apparatus



v.F.aun. 30, 1945. A. v. BEDFORD 2,368,096

PICTURE TRANSMITTING APPARATUS #famme l Paas/nfl ysa/maj:

Alda Bedford Gttorneg jan. 30, 1945. A y- BEDFORD 2,368,096

l PICTURE TRANSMITTING APPARATUS Filed Jan. l5, 1942 '2 Sheets-Sheet 2 lzag-2,9 ma /MMmWe/ F574@ y@ uw M y i I Suventor Alda, VBed 0rd Gttomeg Patenied Jan. 30, 1945 f UNITED STATES :PATENT OFFICE' assauts PICTURE 'rRANsm'r'rING APPAnA'rUs Alda V. Bedford, Collingswood, N. j., asslgnor to Radio Corporation of America, a corporation of Delaware Original application April 20, 1938, Serial No. 203,015. Divided and this application January l5, 1942, Serial No. 426,811

(Cl. P18- 7.2)

8 Claims. rIhis application is a division of application Serial No. 203,015, led April 20, `1938, now Patent No. 2,292,816, granted Augl 11, 1942, in the name of Alda V. Bedford, and entitled Picture transfrequency, that is, direct current. yThe princi- .pal advantage of such transmission is that the v large power tubes of the transmitter are used mor eflciently for reasons which need not be discussed here.

Transmission ofv the direct current and very low 'frequency components of signals cannot be effected conveniently by employing direct current amplifiers inthe entire amplifier channel from the input to the output of the transmitter. As is well known,.it is much easier to design satisfactory A.C. amplifiers than to design satisfactory D.C.amp1iers, especially if the amplitier must give uniform gain over a wide frequency band of the character required in television transmission.

Itis, accordingly, an object of my invention to provide an improved method of and means for transmittingv the direct current and low`frequency components of picture signals.

A further object of my invention is to provide` an improved method of and means for reducing the effects of low frequency disturbances in a picture transmitter.

In the preferred embodiment of my invention, as applied to a television transmitter, horizontal synchronizing impulses are produced at the end of each scanning line and vertical synchronizing or framing impulses are produced at the end of each picture frame. The synchronizingimpulses are mixed with the picture signals for transmis.- sion therewith. l

Suitable means is employed to make the synchronizing impulses have a predetermined amplitude with respect to "black in the picture signal before they, and the picture signals are passed through the main ly-C. amplifier of the transmitter.

- .Y After the signals. have' passed through the main A.'C. amplifier, the synchronizing impulses would no longer have their original amplitude `(because of the loss of the D.C. and low fre-- determined amplitude with respect to "theird inK l employed for restoring their original amplitude and thus restoring the lost components.

In accordance with one of the preferredembodiments of my invention this restoration of l lost components is accomplished by supplying the carrier wave, after it has been modulated by the picture signals and synchronizing impulses, to a biased rectifier circuit. This circuit is so adjusted that when the modulated carrier wave exceeds apredetermined amplitude it is rectified and fed back to a D.C. portion of the transmitter amplifier channel to reduce the amplitude of i the carrier wave. The carrier wave peaks which are rectified are the peaks representing synchronizing pulses. In this way the lsyncluronirg imimpulses are restored to their original amplitude whereby the lost D.C. and low frequency .com-v ponents are restored. The present application relates particularly to the use of additional con'- trol impulses in the restoring circuit.

Other objects, feature and advantages of my scription taken in connection with the accompartying drawings, in which Figure 1 is a. block diagram of a preferred embodiment of my invention,

Figure 2 is a circuit diagram of the embodiment shown in Figure 1,

`Figure 3 is a curve which is referred to in explaining the invention, and

Figures la, 4b and 4c are curves which are referred to in explaining one way in which the apparatus shown in Fig. 2 may be operated.

Referring to Fig. 1, one embodiment of my invention is shown applied to a television transmitter which comprises suitable means represented by the block I for generating picture signals and synchronizing impulses. The apparatus indicatl ed by referencenumeral l may be designed in accordance with the teachings of my Patent 2,192,121, issued February 27, 1940, entitled Television systems and the method of. operation thereof, and assigned to the Radio Corporation of America. The important feature of its design is that the synchronizing signals have a prethe picture. y

The output vof block I is represented by the curve 2. It is a composite signal comprising picture signals 2a', and synchronizing impulses 2b which, in the illustration, are the horizontal synchronizing impulses. The output of block I, in .the example being described, is the same as the output supplied tothe transmitter IGI ofwmy abovementioned patent. T he apparatus in the block- I may be identical with the apparatus shown in the said copending application which supplies a composite signal to the said radio transmitter` It will be noted that in curve 2 each synchronizing impulse consists of a "pedestal which always comes to the black picture level and a delivre date of October 17, 1932, where the peak of each synchronizing impulse goes to a fixed value beyond black in the picture.

The signal supplied by block I is impressed upon an alternating current amplifier 4 which greatly amplies the composite signal. Amplifier 4 is a high gain, wide band pass amplifier which could not easily be designed to passdirect current.

The amplified signal appearing in the output of ampliner 4 no longer contains the direct current and low frequency components. The lost components are restored as follows: The'output of amplifier 4 is supplied toa direct current .ampliiier 36 and next impressed upon a suitable modulator 31 for modulating a carrier wave supplied by an oscillator 38.

The lost direct current and low frequency components are vinserted by means of a feed- -back circuit which derives the negative feedback control voltage from the modulated carrler way'e. The negative feedback circuit includes lsuitable rectiiiers, biasing sources and filters indicated at 39 and a direct current ampliner indicated at 4|.

The preferred embodiment will be better unv derstood by referring to Fig. 2 where parts similar to those in Flg. 1 are indicated by like reference numerals.

Referring to Fig. 2, the amplifier 38 comprises anA amplifier tube 42 which is direct current coupled to the input circuit of the modulator 31 comprising amplifier tubes 43 and 44 confnected in push-pull relation.

In this embodiment of the invention (assumtude of the radiated carrier wave. The carrierwave supplied by the oscillator 38 is also supplied to the input circuit of modulator tubes 43 and 44 whereby the carrier wave is modulated by the video output of the tube 42 in a well known manner. `This modulated carrier wave appears in the tank circuit l, 6 of the modulator whereby it may be suppliedv over a transmission line or the like to the antenna or other lo'ad.

The signalwhich itis desired to radiate .and

Vwhich contains the direct current and low frequency components is illustrated in Fig. 3. It

the modulated carrier wave represents the synchronizing impulses and that all these impulses come to a fixed value with respect to the predetermined black level. It will be understood that without the action of a direct current reinserting circuit, thensignal would not have the character shown in Fig. 3 but instead the carrier wave representative of synchronizing 1mpulses would not be at the black level.

v In accordance with one feature of my invention, I reinsert the direct currentand low frequency components by rectifying the carrier wave representative of synchronizing impulses when it exceeds a certain predetermined value and feeding back this rectified voltage tov the direct current amplifier 28. I 4accomplish this by means of a rectifier 41 which is coupled to the tank circuit 46 through an inductance coil 48. Rectifier 41 and coil are connected in series with a biasing battery, or the like, 49, these' in turn being connected in series through switches 5I and 52 to a filter resistor 83 shunted by a filter condenser 54. 1

With the switches Il and 52 in the position illustrated, for direct current transmission, whenever the carrier wave voltage impressed upon the rectifier l41 exceeds the negative biasing voltage of battery 48 a charging current will fiow into the filter condenser 54. Because of the time constant of the circuit comprising condenser 54 and resistor il, the periodic current flow into condenser 54 caused by recurring synchronizing impulses will be substantially steady direct current. This current may be further filtered by means of a filterI coil 5B and filter resistor 5 1.

'I'he control voltage appearing across resistor 51 is supplied through conductors 58 and l! to the input circuit of the direct current amplifier 4i. .The amplifier 4| comprises tubes 8| and 82 coupled in any suitable manner for transmitting direct current.

The input circuit of tube 6I includes variable biasing means 83 and a switch I4 which is shown in position for direct current transmission. As will be explained later, it may be moved to a position for alternating current transmission,

the control voltage in this case being fed back through a coupling condenser Il.

The output circuit of the tube I! preferably includes a lter comprising a resistor 66 and acondenser 81 for further filtering the feedback control voltage. The plate of the tube l2 is connected through a conductor I8 to the control grid of amplifier tube 42 whereby the direct current output of amplifier 4l biases the tube 42 and thus controls its direct current output.

From the foregoing description it will be seen that, with the switches in the positions illustrated inl Fig. 2, the television transmitter will transmit the direct current and low frequency components for the condition of negative moduis desired for any picture condition. Now, when will be seen that 'the maximum l' amplitude of 75 .a ligh'I signal (as distinguished .from a dark one) modulates the carrier wave, the peak amplitude of the carrier representing the synchronizing impulses increases to cause charging current to flow into condenser l whereby a positive voltage is supplied over conductor 69 to the grid of tube l. This in turn makes the control grid of tube 42 more positive whereby the control grids of tubes 43 and u are made more negative and whereby the peak value of the carrier wave is held' down' to approximately the original pre-set level.

It will be understood that'the above described circuits will remove low frequency disturbances Athe purpose of reinserting D.C. and low frequency components. ,l

It should be noted that the signal represented in Fig. 4a is the one which is radiated to a receiver even though the signal of Fig, 4c is used in the feedback control. Y

It may be noted that additional intelligence or v infomation may be transmitted by modulating which are introduced at points inthe transmitter following the equipment represented by the block l. For example, if .a low frequency disturbance, such as a 60 cycle ripple, occurs in the amplifier t, or in the oscillator 38, it will cause the synchronizing impulses to follow the form of this sine wave and the D. C. reinserting circuit which follows will remove the 60 cycle ripple by again levelling off the synchronizing impulses.

It may be desirable to transmit picture signals n without the direct current component but still retainthe property of my improved system to buck` out low frequency interference and accurately restore low frequency response. To operate the system of Fig. 2 in this manner, switches '5l and 64 are thrown to the dotted line A.`C. po-

condenser S5 which transmits only the A.-C. components. The low frequency component of this alternating current progresses through the negative regenerationfloop and tends to hold the im' pulse peaks to a value which is permitted to drift or change only at a. very low frequency rate.

In accordance with still another embodiment of my invention, the switch 52 may be thrown to the right hand position to produce the feedback signal illustrated in Fig. 4c whereby a control voltage or pilot signal is derived from im-f pulse 98 which are set on top of the pedestals. This type of control may be employed for either AQC. or D.C. transmission.

The signal of Fig. 4c is produced by adding the signal of Fig. 4a (representing one side or polarity of a modulated carrier) which appears in the tank circuit d6 and the signal of Fig. 4b which appears across a resistor el. The side of the 'carrier reprented by Fig. 4c is rectified by the rec-v tifier 41 but because of the biasing batteryfw (or filter l2-J3) only the Peaks of impulses 98 pass thro/ugh the rectifier to charge condenser 5I.

The impulses 96 may be derived from the horizontal synchronizing impulses supplied from unit the resistor 97.

when the control is denvedfrom impulses 96 the pedestals are maintainedv at a fixed level regardless of what happens to the super-sync impulses. 'Ihis would be useful in case the "supersync impulses were subjectedv tosaturation or the impulses 96 before they are impressed across the resistor 91. The effect of such modulation is to introduce an additional modulationA on the transmitted composite signal comprising picture signal and synchronizing impulses. At the television receiver, this modulation is removed before the signal is impressed upon the cathode ray tube by means of a leveling or D.C. reinserting circuit such as that disclosed in P atent 2,252,746,

issued August 19, 1941, to P. W.,Willans. For example, a control tone for a volume expander could be transmitted in this way. It would be taken from a portion of the receiver circuit preceding the leveling or D.C. reinserting stage.

I claim as my invention;

1.-- In combination. means for producing a composite signal comprising picture signals and periodically recurring control signals lwherein said control signals reach a predetermined level when said composite signal contains the desired components and does not contain undesired components, a translating device having an input circuit and an output circuit and being capable of transmitting low frequency components, means for impressing vsaid composite signal upon said input circuit, a degenerative feedback circuit having an input circuit and an output circuit and being capable of transmitting said low frequency components, means for producing periodically recurring control impulses. means for adding said control impulses to the control signals which ap- 40, Dear in the 'output circuit of said translating device to produce a feedback control signal, means for deriving from said feedback control signal a D.C. voltage having a value which is a measure of the amount said periodically recurring control signals depart from said level, means for impressing Said D.C. voltage upon the input circuit of said feedback circuit, and means for coupling the output circuit of said feedback circuit to the input circuit of said translating device.

a condenser in series .said resistor being connected across said source of control impulses, said coupling device being coupled to thev output circuit of said rtranslating device, and a resistor connected yacross said condenser and having such resistance that said condenser discharges only a slight amount between the occurrence yof successive c 'ontrol impulses.

3. The invention according to claim 1 characterized in that additional means is provided for modulating said controlv impulses whereby an additional modulation is placed on said composite signal.

4. In combination, means for producing a composite signal comprising picture signals, pedestals and periodically recurring synchronizing pulses wherein said synchronizing pulses are set on said pedestals and wherein said pedestals reach a predetermined level-'when said composite signal I ,contains the desired components and does not otherl alteration causing them to be (insulted-for l contain undesired components, modulating means for modulating a carrierrwave by said composite signal, said modulating means having an input circuit and an output circuit and being capable of transmitting low frequency components, means for impressing said composite signal upon said input circuit, a degenerative feedback circuit having an input circuit and an output circuit and being capable of transmitting said low frequency components, means for producing periodically recurring control impulses, means for adding said control impulses to the portions of said modulated carrier which are representative of said pedestals and with saidcontrol impulses adjacent to and of greater amplitude than said synchronizing pulses to produce a feedback control signal, means for deriving from said feedback control signal a D.C. voltage having a value which is a measure of the lamount said periodically recurring pedestals depart from said level, means for impressing said D.C. voltage upon the input circuit of said feedback vcircuit', and means for coupling. the output circuit of said feedback circuit to the input circuit of said modulating means. f

5. The method of signal transmission which comprisesproducing a signal which periodically reaches a predetermined level when it contains.

is a measure of the amount saidmodulated ca r-` rier wave portions depart from said level, .and utilizing said D.C. voltagev to cdunteract the departure of said carrier wave portions from said level.

6. The method of signal transmissionwhich comprises producing a signal which periodically reaches a predetermined level when it contains the desired components and does not contain undesired components, amplifying said signal and simultaneously causing an undesired loss of low frequency components, modulating a carrier wave by said amplined signal 'whereby there is 4produced a modulated carrier wave having portions ,representative of saidl periodically recurring signal portions, producing impulses that are of sub-I stantially constant amplitude and which recur signal portions. adding said modulated carrier portions and said impulses to produce recurring control signals, deriving from said control signals a comparatively steady D.C. voltage which is a measure of the amount said modulated carrier wave portions depart from said level, and utilizingV said D.C. voltage to counteract the departure of said carrier wave portions from said level.

7. 'I'he method of signal transmission which comprises producing a signal which periodically reaches a predetermined level 'when it `contains the desired components and does not contain undesired components, amplifying said signal and simultaneously causing an undesired loss of low frequency components, modulating a carrier wave by said amplified signal whereby there is produced a modulatedN carrier wave having portions representative of said periodically recurring signal portions, producing impulses which recur periodically at the same rate as said recurring ,J

signal portions, modulating said periodically recurring impulses whereby an additional modulation is put on the transmitted signal, adding said 'counteract the departure of said carrier wave portions from said level.

8. In combination', an amplifier system having an input circuit and an output circuit, a source periodically at the same rate as saldrecurring of signal containing recurringpulses which depart from a reference voltageby an amount that is proportional to desired components thathave been lost or -undesired components that have been added in circuits preceding said amplifier system, said source being connected to said input circuit, a source of additional pulses modulated by a certain wave, means for combining said additional pulses and a signal derived'from said output cir'cuit in a control circuit with said additionalpulses added to or superimposed upon said iirst pulses, means for producing from said control circuit a pilotsignal which is a measure of the deviation of the peaks of said superimposed pulses from a reference voltage, means for nltering-said pilot signal to produce a comparatively low frequency signal, means for amplifying Mld lowfrequency signal, and means for applying said amplified signal to said input circuit in such phase relation as to reduce said deviation, whereby said certain wave is applied to said input circuit.`

Anna v.-.Bnnr-oRD. 

